In the design of competition bicycles and bicycle parts, weight and stiffness are critical issues. Extremely lightweight structures and structural components are used in the most serious competition bicycles. These lightweight components must be designed for a variety of severe riding environments. This results in a design that must operate at relatively high stresses, close to the strength limits of the materials being used. The demand for a minimum weight bicycle has led the industry into the use of modern, high performance structural materials, such as high strength aluminum, carbon fiber composite and titanium alloys. These high strength materials require more care in the design of fittings and joints because of, a) their susceptibility to fatigue cracking and b) the relatively high load levels at which the fittings and joints are required to operate.
A goal for a bicycle part manufacturer is to eliminate all unnecessary weight from a given part, without compromising its structural integrity and stiffness. There are numerous bicycle suspension forks currently on the market that are not very weight efficient. They have been designed for basic suspension function, without adequate consideration for weight optimization or steering and braking control. Most of the prior art telescoping front fork suspensions fall into this category. These designs tend to be relatively heavy and their stiffness to weight and strength to weight ratios are not very high. They are also relatively flexible laterally and in torsion and cannot provide the stability and accurate steering and braking control for the front wheel assembly that is desired for serious competition cycling. Laboratory tests show that some of the prior art fork designs have torsional spring rates as low as 84 in-lbf/deg and lateral spring rates as low as 140 lbf/in. Some of the heavier steel forks have torsional spring rates in the neighborhood of 230 in-lbf/deg and lateral spring rates of nearly 170 lbf/in, however, their weight exceeds 1500 grams. Based on studies, it has been found that a torsional spring rate in excess of 230 in-lbf/degree and a lateral spring rate in excess of 170 lbf/in is desirable for maximum steering control in competition cycling. The weight of the suspension should be less than 1000 grams.
Most of the prior art fork suspensions use brake arch designs that are inherently too flexible to control wheel wobble and braking action. The name "brake bridge" or "brake arch" says it all. The part was designed and located simply as a support for the brake cable hanger and possibly the brake mounts, similar to the part of the same name used on the rear seat stays of the bicycle. The prior art designs did not realize that the lower sliding tubes need to be rigidly linked to each other in torsion and bending in order to provide top performance of the cantilever brakes and the overall suspension fork assembly.
The present invention uses a unique design for the separate crown structure and brake arch assembly to dramatically increase the strength and stiffness of the fork while reducing weight. The crown structure and the brake arch play key parts in the overall stiffness of the front fork assembly. The invention also provides an improved method of assembly of the various key parts of the suspension fork to reduce manufacturing costs as well as make the system easier to assemble and disassemble for parts and repairs.